Anisotropic Transverse Flow and the HBT Correlation Function

Utilizing the Lorentz invariance of the correlation function we study the effects of anisotropic transverse flow on the HBT correlation function. In particular we show that directed flow would evidence itself by non zero ``side--long'' and ``side--out'' terms in the correlation function. We also show that the study of ratios of the correlation functions evaluated on different event subsamples and/or different Lorentz systems provides information on the source dimensions and velocity which could be less biased by corrections of different kinds. We present a fitting technique appropriate for HBT analysis which does not require binning and is especially useful for multidimensional fitting.